1. Field of the Invention
The present invention is broadly concerned with an improved twin screw extruder especially designed to reduce wear by minimizing the tendency of the screws to separate during rotation thereof and come into wearing contact with the extruder barrel walls; more particularly, it is concerned with such an extruder construction, and a corresponding method, wherein respective, juxtaposed, complemental screw and barrel sections are provided adjacent the outlet end of the extruder in order to provide substantially even distribution of pressure and material resulting in a bearing-type support for the separate screws.
2. Description of the Prior Art
Generally speaking, extruders are industrial devices which include an elongated, tubular barrel, a material inlet at one end of the barrel and a restricted orifice die adjacent the remaining end thereof. One or more elongated, axially rotatable, flighted extrusion screws are situated within the barrel, and serve to transport material along the length thereof. Moreover, the overall extruder is designed to heat, pressurize and render flowable material being processed, typically through the use of high shear and temperature conditions. Extruders have been used in the past to process a wide variety of materials, such as thermoplastic resins and plant-derived materials. In the latter instances, the extruders serve to cook and process the material. A wide variety of plant-derived materials have been processed using extruders, with perhaps the most notable examples being soy, corn and wheat.
One class of extruder in widespread use is the single screw extruder, which includes a single, elongated extruder screw within a substantially circular barrel. Extruders of this type are commonly used for processing plant-derived materials, and have proven over the years to be highly successful. Another general class of extruders are the so-called twin screw machines, which have a pair of juxtaposed elongated, flighted screws within a complemental barrel having a pair of side-by-side, frustocylindrical sections. The screws in such a twin screw machine can be counterrotating (i.e., the screws rotate in an opposite direction relative to each other), or corotating, (i.e. both screws rotate either clockwise or counterclockwise). Twin screw extruders have found wide application in the past, particularly in the plastics industry, although these extruders have also been used for processing of plant-derived materials as well.
One of the chief advantages of a twin screw extruder, as compared with a mono-screw machine, is that the twin screw device operates more in the manner of a positive displacement pump. That is to say, with mono-screw extruders there is considerble fore and aft movement of the material as it progresses along the length of the barrel (such machines can be characterized as drag flow devices), and this can lead to inefficiencies, particularly when extremely viscous materials are being processed. In the case of a twin screw machine though, this fore and aft "slippage" of material during processing is substantially reduced or eliminated. Thus, in handling extremely viscous material such as synthetic resins or the like, twin screw extruders are normally the apparatus of choice.
Despite these advantages however, twin screw extruders have presented severe operational problems in their own right. Perhaps the most significant problem in connection with the twin screw machines in the fact that they exhibit a marked tendency to prematurely wear out machine components. Specifically, with a twin screw machine, build-up of pressures at the region where the screws are intermeshed develops outwardly directed forces which tend to separate the screws and effectively push the screws into wearing contact with the adjacent barrel walls. This in turn leads to rapid wear of the screw and barrel components, with the result that maintenance costs and the down time are increased. Indeed, it is not unknown in the extruder art to hear a twin screw extruder "rumble" by virtue of the screws coming into undue rubbing contact with the barrel walls during operation.
Another problem sometimes encountered with twin screw extruders is the velocity differential developed in the material at the outboard regions of the extruder screws, as compared with the regions where the screws are intermeshed. That is to say, material passing along the extruder adjacent the outboard regions of the screw tends to move at a faster rate than does material passing along the extruder at the region where the screws are intermeshed. This can be most graphically seen at the outlet of the extruder, where material will pass through outboard die apertures at a greater rate than through the central apertures. As can be appreciated, such a differential velocity is to be avoided, inasmuch as it can lead to uneven cooking and flow conditions within the extruder. In the past, attempts have been made to eliminate this differential velocity problem by provision of elongated die spacers between the ends of the screws and the actual extrusion dies. While this does tend to decrease the velocity differential, use of such die spacers can lead to dead spots or areas of stagnation and consequent burning or scorching of material being processed. This problem is most critical in the extrusion of foodstuffs or another biological materials.
Russian Pat. No. 410969 describes a twin screw plastics extruder having a short, unflighted bullet affixed to the foward end of each screw. This construction is deemed deficient for a number of reasons, most especially because the smooth, unflighted bullets of the Russian patent do not provide any positive transport of material along the bullet length, and further may not give substantially even distribution of material and pressure around the peripheries of the bullets.
Accordingly, while twin screw extruders have undeniable advantages, they also exhibit several significant disadvantages which have tended to limit their utility.